1. Field of the Invention
The present invention relates to video coding and decoding devices. More particularly, the invention is directed to a video coding device for hierarchically encoding a video sequence, and a video decoding device for decoding signals coded by the video coding device.
2. Description of Related Art
With recent developments of multimedia services, the image processing technique gets an increasing importance arid various kinds of studies have been made in various fields of applications.
In general, video information contains a very large amount of information. Accordingly, it is impossible to practically transmit video information as it is, since a transmission line with a very wide-band transmission capacity is required. On the other hand, a video signal contains redundances which need to be reduced. Therefore, a coding or a compression coding technique is widely used for treating video signals at high efficiency.
In a conventional video coding device using an interframe predictive orthogonal transform coding method, a motion-compensated interframe-predicting portion encodes input video signals per frame by motion-compensated interframe-predicting method and outputs motion-vectors. Namely, the motion-compensated interframe-predicting portion reads a preceding video frame image, predicts a motion-compensated video frame image from a current input video frame image and the read preceding video frame image and outputs a predicted video signal, which was encoded, decoded and then stored in a frame memory portion. A difference calculating portion determines a prediction error signal that is a difference between the input video signal and the predicted video signal read from the frame memory portion, thus eliminating temporal redundancy from the signal to be encoded.
The prediction error signal outputted from the difference calculating portion is transferred to an orthogonal transforming portion whereby said signal is orthogonally transformed and removed off spatial redundancy. In consequence of this processing, a transformation coefficient is outputted.
The transformation coefficient from the orthogonal transforming portion is quantized and encoded with a compressed amount of information by a coding portion.
The output signal from the coding portion is transferred as coded information to an external circuit and a decoding portion.
The decoding portion performs processing operations in a reverse fashion as compared to those performed by the coding portion and outputs the transformation coefficient to a reverse orthogonal transforming portion that in turn conducts reverse orthogonal transformation of the received transformation coefficient.
The output signal from the reverse orthogonal transforming portion is added to the predicted video signal read from the frame memory portion by an adding portion. The resultant signal is stored in the frame memory portion and will be used for interframe prediction of a next input video signal.
Input video signals are thus encoded by a loop structure (coding loop).
As described above, the conventional video coding device is capable of efficiently encoding video sequence by previously eliminating temporal redundancy of image information through motion-compensated prediction and spatial redundancy through orthogonal transformation. However, the conventional coding method encodes a video sequence frame by frame and, therefore, coded image information by the conventional coding method cannot be correctly decoded for a whole image if an image signal should be subjected to a transmission error or loss of information in a transmission line.
Furthermore, the conventional method uses the interframe prediction coding technique, and therefore incurs a problem of handing down an incorrectness occurred in a prior decoded image to all subsequent frames to be encoded.
These problems may be solved by applying a hierarchical coding method that arranges image signals in a hierarchy and encodes signals of each hierarchical layer separately from each other. Namely, when incorrect decoding occurred in a hierarchical layer of image signals, this method can minimize the impairment of decoded images by only decoding signals of other correct layers.
In a hierarchical video coding device using divided frequency bands, similarly to the above-described conventional method, an input video sequence is encoded frame by frame by a motion-compensated interframe-predicting portion and a prediction error signal per frame is outputted from a difference calculating portion.
The prediction-error signal from the difference calculating portion is divided into a plurality of frequency bands and outputted in bands respectively by a band-dividing portion. The band-components of the prediction error signal, which were outputted from the band-dividing portion, are encoded separately from one another by coding portions respectively. Encoded information is outputted to an external circuit and corresponding decoding portions.
The decoding portions performs processing operations reverse to those performed by the coding portions from which respective band components of the prediction-error signal are outputted.
On the other hand, a predicted image signal read from a frame memory is divided into frequency bands and outputted as respective frequency-band-components by a band dividing portion.
The frequency-band-components of the predicted image signal from the band dividing portion are added to the corresponding frequency-band-components of the prediction-error signal from the decoding portions by an adding portion respectively. The obtained frequency-band-components of a decoded image signal are outputted respectively.
These frequency-band-components of the decoded image signal are synthesized to form a decoded image signal (through processing operations reverse to those performed by the band dividing portion) by a synthesizing portion. The decoded image signal is stored in the frame memory portion and will be used for interframe prediction of a next input image signal.
According to the above-mentioned processing method, it is possible to minimize degradation of a video sequence due to an error or a loss in information transmitted over a transmission line since a decoding error occurred in a certain frequency-band-component may be confined in said frequency band.
However, when transmission error and/or a loss of information occurs in a transmission line, the above-mentioned conventional hierarchical coding device can enclose the decoding error within a hierarchical layer but cannot completely prevent said error from propagating to subsequent images.